1. Field of the Invention
The present invention relates to a constant voltage circuit which is applied to a power supply device, such as a power adapter, and more particularly to a constant voltage circuit for a power adapter which is capable of varying an output voltage with loads so as to reduce the charging time of a system battery.
2. Description of the Related Art
In general, a power adapter, which is adapted to supply a constant voltage for charging of a battery, supplies the constant voltage irrespective of loads, as will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram of a conventional constant voltage circuit.
Referring to FIG. 1, the conventional constant voltage circuit comprises a transformer 10 having a primary coil and secondary coil and acting to transform a commercial alternating current (AC) voltage according to a turn ratio of the primary and secondary coils, a rectifier 20 for rectifying an output voltage from the transformer 10, a voltage detector 30 for detecting the output voltage rectified by the rectifier 20 to output a detected voltage Vd, a comparator 40 for comparing the detected voltage Vd from the voltage detector 30 with a reference voltage Vref to output a difference voltage therebetween, a signal coupler 50 for performing a coupling operation for a signal corresponding to the difference voltage from the comparator 40, and a controller 60 for controlling a primary voltage of the transformer 10 in a pulse width modulation (PWM) manner in response to an output signal from the signal coupler 50.
In this conventional constant voltage circuit, the output voltage from the transformer 10 is detected and the comparison is made between the detected voltage Vd and the internal reference voltage Vref (about 2.5V) to generate a difference voltage therebetween. The controller 60 controls a turn-on duty of a switch, for example, a field effect transistor (FET), connected to the primary coil of the transformer 10 on the basis of the difference voltage, so as to maintain the output voltage constant.
However, the above-mentioned conventional constant voltage circuit has a disadvantage in that the output voltage is controlled to be outputted constantly irrespective of loads, and more particularly even when a battery is in an uncharged state, thereby making it impossible to rapidly charge the battery.